"Holiest" is a very vague word.
Look, we have a tradition that a few people in the Bible, like Benjamin or Kilav, who was one of King David's sons who didn't inherit the throne and just sat around the palace, never sinned at all. But they didn't do all that much interesting either. No great heights, no great falls. What do you call that? "Holy"? Depends how you use the term.
The Talmud even tells the story of Moses time-traveling to the classroom of Rabbi Akiva and not understanding everything going on there! Moses' #1 job was transmitting the message, not necessarily analyzing it.
The Torah concludes with the observation that he was the greatest prophet. And that's one of Maimonides' 13 Principles of Faith.
The point is that he was the one to take G-d's dictation in recording our law. This law includes how to deal with all future prophets. If a later prophet could have greater prophecy than Moses, he could say "Moses' prophecy is now obsolete, and here are new laws." And it's a fundamental premise of our faith that that can't happen.
We can talk about how Moses achieved this level of prophecy, or why G-d chose him, or the like; but the point is he's the supreme prophet.